In mass spectrometers used, for example, for proteome analysis, high sensitivity, high mass accuracy and MSn analysis, etc. are required. Description is to be made simply how such analysis has been conducted so far.
As a high sensitive mass spectroscopy capable of MSn analysis, a quadrupole ion trap mass spectrometer is known. The basic operation principle of the quadrupole ion trap mass spectrometer is well-known (for example, refer to Patent Document 1: U.S. Pat. No. 2,939,952 (prior art 1)). The quadrupole ion trap includes a pole trap comprising a ring electrode and a pair of endcap electrode, and a linear trap comprising four quadrupole rod electrodes. By applying a high frequency voltage at a frequency of about 1 MHz to the ring electrode or between the quadrupole rod electrodes, stable conditions are attained for the ions with a predetermined or more of mass number in the quadrupole ion trap and they can be accumulated.
Further, MSn analysis in the pole trap has been reported (Patent Document 2: U.S. reissued Patent No. 34,000 (prior art 2)). In this method, ions generated from the ionization source are accumulated in the pole trap and precursor ions having a desired mass are isolated. After ion isolation, a supplemental AC voltage resonant to the precursor ions is applied between the endcap electrodes, thereby extending the ion orbit, colliding the same against a buffer gas filled in the ion trap to dissociate the ions. The fragment ions are successively ejected under sweeping of the ring voltage and detected. Since the fragment ions show an inherent spectral pattern depending on the difference of molecular structure of the precursor ions, more detailed structural information for the sample molecule can be obtained.
Further, MSn analysis in the linear trap region comprising four quadrupole rod electrodes has been reported (refer to Patent Document 3: U.S. Pat. No. 5,420,425 (prior art 3)). While the trapping efficiency for the externally generated ions is 20% or less in the pole trap, the linear trap has an advantage that the trapping efficiency is approximately 100%. According to this method, ions generated from the ionization source are accumulated in the ion trap region and precursor ions having a desired mass are isolated. After ion isolation, a supplemental AC voltage resonant to the precursor ions is applied between opposed pair of quadrupole rod electrodes. Thus, the ion orbit is expanded and abutted against the buffer gas filled in the linear trap region to thereby dissociate the ions. The fragment ions are ejected successively under sweeping of the ring voltage and detected. Since the fragment ions show an inherent spectral pattern depending on the difference of the molecular structure of the precursor ions, more detailed structural information for sample molecules can be obtained. Since the method has higher efficiency for taking in ions from the outside and less undergoes the effect of space charges compared with the method of the prior art 2, it is highly sensitive.
A method of enabling high mass accuracy and MSn analysis by using a linear trap region has been reported (refer to Patent Document 4: U.S. Pat. No. 6,020,586 (prior art 4)). According to this method, MSn analysis is possible by repeating the ion isolation and ion dissociation in the linear trap region like the prior art 3. Ions are introduced from the linear trap region to the acceleration region of the time-of-flight mass spectrometer in the axial direction by applying a DC voltage to the electrodes before and after the linear trap region. By arranging the direction of ion introduction and the direction of acceleration orthogonal to each other, extension for the position in the direction of acceleration and the energy can be suppressed. As a result, higher mass accuracy than that in the prior art 3 can be attained.
Further, improvement for the sensitivity in the prior art 4 has been reported (refer to Patent Document 5: JP-A 526447/2001 (prior art 5)). According to this method, it is stated that analysis at higher sensitivity than in the prior art 4 is possible by arranging linear trap regions in two stages and share the role of accumulation, isolation and dissociation on the first stage and the second stage respectively.
Further, a method of enabling high mass accuracy and MS/MS analysis has been reported (refer to Non-Patent Document 1: H. R. Morris, et al., Rapid Communication in Mass Spectrometry, 1996, Vol. 10, p. 889 (prior art 6)). According to this method, ions selected for the mass in a quadrupole mass spectrometer are accelerated and introduced into a collision chamber. Incident ions collide against the buffer gas in the collision chamber and are dissociated in the collision chamber. An Ar gas at about 1 to 10 Pa is supplied into the collision chamber, in which multipole electrodes are disposed. The dissociated ions are converged by the multipole electric fields and collision with the buffer gas near the central axis and then introduced to the time-of-flight mass spectrometer and detected. This enables MS/MS analysis.
Further, a method for improving the sensitivity in the prior art 6 is described in Patent Document 6 (refer to Patent Document 6: U.S. Pat. No. 6,507,019 (prior art 7)). According to this method, a voltage on the outlet of the collision chamber is controlled in synchronization with the timing of applying an acceleration voltage in a time-of-flight mass spectrometer thereby improving the sensitivity for ions in a specified range of mass number.
Further, a method for improving the sensitivity in the prior art 6 has been reported (refer to Patent Document 7: U.S. Pat. No. 6,504,148 (prior art 8)). According to this method, ions of a predetermined mass number trapped to quadrupole rods can be ejected axially by using a supplemental AC voltage and introduced to a collision chamber or a time-of-flight mass spectrometer. This can improve the ion duty efficiency in precursor scanning and neutral loss scanning to greatly improve the sensitivity in the measuring mode.
The methods of the prior arts 1, 2, and 3 involve a problem that the mass accuracy obtainable is only about 10 ppm to 100 ppm by the chemical mass shift caused by collision against a buffer gas upon ion detection and space charges caused by coulombic repulsion between ions to each other and it can not be applied to the application field requiring high mass accuracy.
The coupling system for a linear trap region and a time-of-flight mass spectrometer in the prior arts 4 and 5 involves the following problem. The ion ejection time from the linear trap region to the time-of-flight mass spectrometer lowers the ion duty efficiency (duty cycle) and, thus, lowers the sensitivity since other measurement is interrupted during the ion ejection. In order to avoid lowering of the duty cycle, it is necessary to decrease the ejection time for the ions from the linear trap region to the time-of-flight mass spectrometer. For this purpose, it is necessary to increase the ejection potential for the ions from the linear trap region. Use of a high ejection potential results in a problem of increasing the divergency of energy in the direction of acceleration in the time-of-flight mass spectrometer and, as a result, this lowers the mass resolution power. That is, the method of the prior art 4 and 5 has a problem that the sensitivity and the resolution power can not be compatible.
In the method of the prior art 6, 7, and 8, MSn (n≧3) analysis is impossible and it is efficient for the identification of molecular ions of high mass. Further, it has a problem that the ion dissociation proceeds in multi-stages such that dissociated ions after entering the collision chamber are further dissociated, which are then dissociated further and it is sometimes inefficient to presume the original ion structure from the fragment ions.
As has been described above, it is impossible to obtain a mass spectrometer capable of providing high sensitivity and high mass accuracy, and MSn (n≧3) analysis in the prior art.